The present invention relates to a brake disk of the ventilated type which has a large number of air ducts defined in a disk portion having opposite sides clamped between friction pads with the air ducts extending radially through the disk portions, and a plurality of joining portions formed on a mounting flange portion at circumferentially spaced apart distances and joined to a wheel hub by bolts.
A brake disk of such type is already known, for example, as disclosed in Japanese Patent Application Laid-Open No. 156736/83.
In such a brake disk, if a strain is produced in the disk portion by frictional heat due to the contact of the disk portion by the friction pads, a phenomenon known as "judder" is generated so that the friction pads are oscillated from the disk portion during braking. This may cause the braking feeling to be degraded.
The present inventors have cleared up the following matter as one factor of the thermal strain of the disk portion: When the disk portion has been heated by frictional heat, that area of the disk portion which is nearer to the joining portions to the wheel hub is subjected to the transfer of the heat through the joining portions to the wheel hub, in addition to the cooling by the air flowing through air ducts. Hence, such an area is better cooled than an area farther away from the joining portions. Thus, a large difference in temperature is produced between the areas of the disk portion nearer to and more remote from the joining portions. This causes a thermal strain.